Device to prevent sodium freezing around shaft penetration

ABSTRACT

A DEVICE FOR PREVENTING SODIUM VAPOR FROM FREEZING AROUND A MOVABLE SHAFT EXTENDING INTO A POOL OF HOT SOLIUM THROUGH A COVER GAS PROVIDED THEREABOVE. THE DEVICE CONSISTS OF A SEALED HOLLOW-WALL TUBE CONTAINING SODIUM POSITIONED AROUND THE SHAFT IN THE REGION OF POSSIBLE FREEZING WITH ITS LOWER ENDIN HEAT EXCHANGE CONTACT WITH THE COVER GAS. MEANS MA BE PROVIDED FOR PASSING INERT GAS THROUGH THE ANNULUS BETWEEN THE SHAFT AND THE HOLLOW-WALL TUBE COUNTERCURRENT TO SODIUM VAPOR RISING FROM THE POOL OF HOT SODIUM.

J. H. GERMER DEVICE TO PREVENT SODIUM FREEZING AROUND SHAFT PENETRATION Filed Feb. 18, 1971 2 Sheets-Sheet 1 E m w E Z M U L 4 2 w w m F 1 R PM for Jl L' niJQ M H III I I I I]. P

3 I l w m Fig- DEVICE To PREVENT SODIUM FREEZING AROUND SHAFT PENETRATION Filed Feb. 18, 1971 J. H. GERMER Oct. 24, 1972 2 Sheets-Sheet 2 A r w z m M WW T W 6M Ma i Z w m 2 E I u mg 5 M E 2 .1 EMS NWT 4 I -..:-:i-::i-l!-.w I 0 6 w 4 a 0 u/ 0 4w 4 w M// J p &? 7 7/ 4 n T w J I w, H i /Mn M 4m TWIIATMKTI W/KMV,JJITIQJTDJT. y a%/ H 7 i 2, y L W 4 la United States Patent O 3,700,551 DEVICE TO PREVENT SODIUM FREEZING AROUND SHAFT PENETRATION John H. Germer, San Jose, Calif., assignor to the United States of America as represented by the United States Atomic Energy Commission Filed Feb. 18, 1971, Ser. No. 116,575 Int. Cl. G21c 1/02 US. Cl. 176-40 10 Claims ABSTRACT OF THE DISCLOSURE A device for preventing sodium vapor from freezing around a movable shaft extending into a pool of hot sodium through a cover gas provided thereabove. The device consists of a sealed hollow wall tube containing sodium positioned around the shaft in the region of possible freezing with its lower end in heat exchange contact with the cover gas. Means may be provided for passing inert gas through the annulus between the shaft and the hollow-wall tube countercurrent to sodium vapor rising from the pool of hot sodium.

CONTRACTUAL ORIGIN OF THE INVENTION The invention described herein was made in the course of, or under, a contract with the United States Atomic Energy Commission.

BACKGROUND OF THE INVENTION This invention relates to means for preventing vapors of a liquid metal from freezing around a movable shaft which extends into the liquid metal. In more detail, the invention relates to means for preventing sodium from freezing around a control rod shaft for a sodium-cooled fast breeder nuclear reactor.

A sodium-cooled reactor is ordinarily provided with an inert gas (such as argon) over the sodium in the region above the reactor. It is generally referred to as a cover gas. The cover gas is at essentially the same temperature as the reactor outlet sodium, about 1200 F. At this temperature, a considerable amount of sodium vapor will be present in the cover gas.

Various penetrations, in particular control rod drive shafts, extend through the reactors upper shield, through this cover gas space, and down into the sodium.

It is desirable to operate the upper ends of these shafts at a considerably lower temperature than the cover gas since the control drive is located at the upper end of the shaft. The resulting temperature difference in the annular spaces surrounding these shafts is such that a considerable amount of naturally convecting hot, vaporladen gas will tend to circulate in the annulus. This presents a serious problem when sodium vapors condense and freeze in the annulus.

The present invention consists of a device which prevents sodium from freezing in this annulus. The device assures that the surfaces of the shaft with a temperature below the freezing point of sodium are located in regions that are not easily accessible to sodium vapor diffusion. The invention will, of course, also apply to systems employing other liquid metals in a similar way.

BRIEF DESCRIPTION OF THE DRAWINGS The device may be better understood by reference to the drawings in which:

FIG. 1 is a partial sectional view showing a control drive shaft penetrating the shield, cover gas and liquid sodium of a sodium-cooled nuclear reactor and disclosing one embodiment of the present invention.

3,700,551 Patented Oct. 24, 1972 FIG. 2 is a graph showing the temperature along the control drive shaft according to this embodiment of the invention.

FIG. 3 is a partial sectional view disclosing an alternative embodiment of the invention and FIG. 4 is a graph showing temperatures according to this embodiment of the invention.

As shown in FIG. 1 a control rod drive shaft 10 penetrates upper shielding 12 of a sodium-cooled fast breeder reactor and extends through a cover gas 14-such as argon-into a pool 16 of liquid sodium submerging the core of the reactor. Annulus 18 between shaft 10 and shielding 12 is wide enough to accommodate a metallic tube 20 having a solid upper portion 22 and a lower portion 24 having a cavity 26 therein. Lower portion 24 is nearly full of sodium 28 leaving an expansion space 30 above the sodium. Also a cooling coil 32 surrounds the solid portion 22 of the tube 20 keeping the temperature at this point below the freezing point of sodium.

As is well known, the heat transfer properties of liquid sodium are such that heat addition to the lower end of a sodium-filled vessel is readily transferred to the upper end. For this reason, a very large heat flow would be required to cause the lower end of the sodium in cavity 26 to rise in temperature much above the temperature of the upper end and the sodium can be considered essentially isothermal. Since the thermal resistance of the walls of tube 20 which serve to conduct heat from the sodium 28 contained in cavity 26 to the cooling coils 32 is lower than the thermal resistance at the lower end of tube 20 between sodium 28 and the hot cover gas 14, the temperature of the sodium 28 is substantially less than the average of the temperatures of the hot cover gas 14 and the cooling coils 32 which would be the temperature of the sodium if these thermal resistances were equal. The difference in the thermal resistances is due to the relatively poor heat transfer characteristics of argon gas compared to those of the metal-such as stainless steelof which tube 20 is formed.

FIG. 2 indicates the temperature profile that results. Two features of importance will be noted.

(A) The temperature in the annulus 18 is above the freezing point of sodium except near the upper end at a remote distance from the hot cover gas 14.

(B) The portion of the annulus below that which is below the freezing point of sodium is all at a low temperature relative to the hot cover gas. For this reason, sodium vapor in the hot cover gas entering the lower end of the annulus will have ample opportunity to condense and run downward, thus very little of this sodium vapor will ever reach the region where freezing will occur.

One feature of this arrangement is that the major portion of the length of the device cannot cool to a tempera ture below the freezing point of sodium by removing heat at the upper end, for instance by cooling coil 32. If the upper end of tube 20 were to cool to below the freezing point of sodium, it would freeze and increase in thermal resistance due to suppression of natural convection, thus causing heat from the lower end to raise the temperature. For this reason the device acts as a thermostat to prevent the possibility of reaching the freezing point in a region of high sodium vapor content in the gas.

A second embodiment of the invention is shown in FIG. 3. As shown in FIG. 3, a control rod drive shaft 40 penetrates the upper shielding 42 of a sodium-cooled fast-breeder reactor through an aperture 44 and extends through the hot cover gas 46 into the liquid sodium 48 within which the core of the reactor is submerged. The diameter of the aperture 44 is such that there is a narrow, annular space 50 between the shaft 40 and shielding 42, except at the lower end of the shield 42. In this region the diameter is increased to receive a sodiumfilled hollow-wall tube 52.

Tube 52 has a totally enclosed, hermetically sealed cavity 54 containing sodium 56 and a void space 58 for sodium expansion. The outer surface 60 of tubular member 52 is in close fitting contact with the shield 42 While the inner surface 62 defines an annular space 64 about the shaft 40 similar to the space 50 above. The lower end 66 of tube 52 extends into the hot cover gas 46 below the bottom of shield 42 while the upper end 68 is spaced from a shoulder 70 connecting the two diameters of aperture 44, thus forming an open area 72 into which a cool inert gas is fed through inlet 74.

This cool gas passes downward through the annular space 64 where it picks up heat from the surface 62 of tube 52 and discharges at exit 76 to mix with the hot cover gas 46. The outer surface 60 of tube 52, located within the area of the hot inert gas 46, is provided with fins 78 to improve the heat transfer from the hot gas 46 to the sodium 56, while helical grooves 80 located in the inner surface 62 of tube 52 tend to improve the heat transfer from the sodium 56 to the gas in annulus 64.

As has been stated heat addition to the lower end of a sodium-filled vessel is readily transferred to the top. Thus, looking at the graph of FIG. 4, it can be seen that, with the liquid sodium 48 and cover gas 46 being at about 1200 F., the temperature of the sodium in tube 52 is constant except where the cool gas enters. As shown in the left-hand curve, the cooling gas is admitted through inlet 74 at a temperature below the freezing point of sodium. In annular space 64, this gas heats up almost instantly, above the freezing temperature of sodium, and its temperature gradually increases until, at exit 76 of the annulus 64, it is close to the temperature of the sodium 56 in tube 52.

Thus it is seen that the cool argon entering through inlet 74 will prevent hot gas from entering annulus 44. This is desirable since the control drive operates in the region of the upper end of annulus 44. Because of the presence of tube 52 the temperature of the argon passing downwardly through annulus 64 is above the freezing point of sodium except for a small region near the upper end of annulus 64. Thus, in order for sodium to freeze it would have to diffuse against the stream of argon over most of the length of the annulus.

It will be observed that satisfactory results could not be attained merely by the introduction of a gas through inlet 74 at any temperature. If a hot gas in introduced it will prevent sodium from freezing but it would be difficult to maintain the upper end of annulus 44 at a low temperature due to natural convection currents. If a cold gas is introduced most of annulus 64 would be at the temperature of the inlet stream of argon and severe condensation and freezing of sodium would occur near the lower end of annulus 64 as a result of back diffusion from the hot argon gas 46.

It will be understood that the invention is not to be limited to the details given herein but that it may be modified within the scope of the appended claims.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A device for preventing liquid metal vapor from freezing around a movable shaft extending into a pool of liquid sodium through a cover or shield provided thereabove, there being a cover gas present in the space between the pool of liquid sodium and the cover, said device comprising a sealed hollow-wall tube containing liquid sodium having an expansion space at the top of the hollow positioned around said shaft and spaced therefrom in at least the lower portion of said cover with its lower end in heat exchange contact with the cover gas.

2. A device according to claim 1 and including means for passing an inert gas downwardly through the annulus between said shaft and said hollow-wall tube into the cover gas.

3. A device according to claim 1 wherein said hollowwall tube includes a solid portion at the top thereof and means for cooling said solid portion.

4. In a fast breeder nuclear reactor including a core submerged in a pool of liquid sodium wherein a plurality of movable shafts penetrate to the pool of sodium through a shield disposed above the reactor core and wherein a cover gas fills the space between the pool of sodium and the shield, the improvement comprising a sealed hollowwall tube containing sodium surrounding each movable shaft and spaced therefrom as it passes through at least the lower portion of the shield, the lower end of said hollow-wall tube being in heat exchange contact with the cover gas.

5. The improvement of claim 4 wherein said hollowwall tube forms the lower portion of a metal tube which surrounds the shaft in the region of the shield and including means for cooling the solid portion of said metal tube.

6. The improvement of claim 5 wherein the thermal resistance of the walls of the expansion space for the sodium in said hollow-wall tube is lower than the thermal resistance of the wall between the sodium in the hollowwall tube and the cover gas at the lower end of the hollow-wall tube.

7. The improvement of claim 4 and including means for passing an inert gas downwardly through the annulus between the shaft and the hollow-wall tube.

8. The improvement of claim 7 wherein the hollowwall tube extends below the bottom of the shield into the cover gas.

9. The improvement of claim 8 wherein the extension of the hollow-Wall tube contains a plurality of circumferential metal fins.

10. The improvement of claim 9 wherein the inner wall of the hollow-wall tube has a helical groove therein facing the movable shaft.

References Cited UNITED STATES PATENTS 3,296,085 1/1967 Peck et al 176-40 REUBEN EPSTEIN, Primary Examiner US. Cl. X.R. -105 

